(1) Field of the Invention
The present disclosure is related to an ultrasound signal processing device, and an ultrasound diagnostic device utilizing the ultrasound signal processing device. In particular, the present disclosure relates to beam forming in an ultrasound signal processing device.
(2) Description of the Related Art
Typically, an ultrasound diagnostic device transmits ultrasound towards the inside of a subject via an ultrasound probe (referred to in the following as a “probe”), and receives reflected ultrasound (an echo) via the probe. The reflected ultrasound is generated within the subject due to different tissues in the subject having different acoustic impedances. Further, an ultrasound diagnostic device generates an ultrasound tomographic image based on electric signals acquired through the reception of the reflected ultrasound, and displays the ultrasound tomographic image on a monitor (referred to in the following as a “display unit”). The ultrasound tomographic image shows the structures of tissues inside the subject. Ultrasound diagnostic devices are widely used for the shape diagnosis of living bodies, for having low invasiveness and achieving real-time observation of tissues through tomographic images and the like.
A typical method applied in conventional ultrasound diagnostic devices for reception beam forming (i.e., forming signals based on received reflected ultrasound (echo signals)) is so-called delay-and-sum (DAS) beam forming. One example of delay-and-sum beam forming can be found disclosed in pages 42-45 of “Ultrasound Diagnostic Device”, written by Masayasu Itou and Tsuyoshi Mochizuki and published by Corona Publishing Co., Ltd (Aug. 26, 2002).
FIG. 16 is a schematic illustrating reception beam forming in a conventional ultrasound diagnostic device. The conventional ultrasound diagnostic device illustrated in FIG. 16 includes a probe 201 and a reception beam former 202. The probe 201 includes a plurality of ultrasound transducer elements (referred to in the following as “transducer elements”) 201a that receive ultrasound reflection (echo signals) from the subject. The reception beam former 202 electrically converts the reflected ultrasound received by the transducer elements 201a into analog electronic signals, converts the analog electronic signals into digital electronic signals through some amplification and A/D conversion, and performs delaying and summing of the digital electronic signals. The reception beam former 202 includes a plurality of delaying units 2021, and an adding unit 2022. The delaying units 2021 are each associated with a corresponding one of the transducer elements 201a, and performs amplification, A/D conversion, and delaying with respect to an electric signal. The adding unit 2022 provides signals output from the delay units 2021 with weights referred to as so-called apodization weights, and sums the weighted signals. The reception beam former 202 generates and outputs an acoustic line signal for a measurement point located along the central axis of the transmitted ultrasound beam. In specific, the reception beam former 202 generates the acoustic line signal by each of the delaying units 2021 performing delaying with respect to an electric signal based on reflected ultrasound obtained by the corresponding transducer element 201a, and by the adding unit 2022 summing the delayed electric signals obtained from the delaying units 2021. Typically, the delay amount that each delaying unit 2021 applies is based on the distance between the corresponding transducer element 201a and the measurement point, which is located along the central axis of the transmitted ultrasound beam as discussed above.
In specific, suppose that: P denotes a measurement point that is at a given depth inside the subject and that is located along the central axis of the transmitted ultrasound beam; C denotes a transducer element, among the plurality of transducer elements, that is closest to the measurement point P; dc denotes the distance between the measurement point P and the transducer element C; m denotes a transducer element other than the transducer element C; dm denotes the distance between the measurement point P and the transducer element m; and cs denotes ultrasound velocity. Here, the time point at which reflected ultrasound from the measurement point P arrives at the transducer element m is later than the time point at which reflected ultrasound from the measurement point P arrives at the transducer element C by (dm/cs−dc/cs). Thus, by calculating the time point at which reflected ultrasound from the measurement point P arrives at the transducer element C, the time point at which reflected ultrasound from the measurement point P arrives at the transducer element m can be calculated based on this difference (dm/cs−dc/cs). As such, each delaying unit 2021 specifies a reception signal for the corresponding transducer element 201a by considering the difference between arrival times of reflected ultrasound, and the adding unit 2022 generates an acoustic line signal by summing the reception signals specified by the delaying units 2021.